They're bad for users because they tend to use the same weak password across multiple sites which means they're only as safe as the least secure site they use. They're bad for developers because the sign-up process loses a large portion of potential users. They also force every developer to jump through all the steps required for a world-class identity system:

Social login isn't a panacea but in the long run the only viable solution is delegating authentication to a small set of high quality identity providers. It has to be a small set to avoid the damage to conversion rates caused by the NASCAR problem. They will be high quality since the market is so competitive that low quality providers (where quality is a measure of the experience/value provided to users, developers and publishers) will find it hard to acquire and retain users. The market will be competitive simply because various entities have realised that social login is the backbone of any successful ecosystem so they're making the necessary investments.

This is sub-optimal but the OpenId dream (where every user runs their own server and their own OpenID endpoint) ran aground on the twin rocks of user apathy and security. Even if the dream had survived that it still didn't have a good answer to the major publishers who wanted to know what they would be getting in return for the extra effort of supporting OpenID. If you think OpenID Attribute Exchange and PAPE are solutions then you may be wearing the complicator's gloves.

The only questions left are:

who will be these identity providers

what will be their business models

how will we assess and choose between them

how will we keep them honest

how much control do they give users

do they help developers build better and more valuable services as time passes

will they become gatekeepers that constrain future innovations

This moves us to a world where users authorise developers rather than particular apps or web sites. As a result once you give a developer access to your information you give all of their services and apps access to your information. Technologies like OAuth2's bearer tokens mean that developers can easily pass access to a user's information back and forth between their mobile apps and their back-end systems.

In this new world developers will have to deal with multiple competing identity providers who each impose their own constraints and policies in order to protect their users. As a result developers will have to start thinking in a more sophisticated way about the way they propagate identity between their different systems, track the provenance of user data and honour the conflicting policies imposed by multiple identity providers. They'll also need more nuanced terminology. It won't be enough to think solely in such crude terms as "public" versus "private". Developers will also have to be aware of the subtle distinctions between "obscure" versus "secret" and "public" versus "publicised".